Robustness: webhook rate limiter records a delivery before it succeeds

[LarsLaskowski]

Source: Full code review 2026-07-17 (docs/reviews/2026-07-17-full-code-review.md), finding F5.
Category: robustness · Severity: low · Effort: small

Problem

rateLimited (internal/alert/notify.go:199-209) stamps lastSent[key] = ev.At at check time, before deliver runs. If that delivery then fails all retries, the next firing of the same metric within notify_min_interval_seconds is still suppressed as a "repeat" — even though nothing was ever delivered.

Suggested fix

Stamp lastSent only after a successful post (or clear the stamp when deliver gives up), so the limiter counts deliveries, not attempts.